Optical waveguide devices have various applications. Devices that function as frequency shifters have many applications. Such devices can be used in laser heterodyne systems, in phase nulling fiber optic gyroscopes, etcetera.
Most applications for frequency shifters employ acousto-optic (Bragg) frequency-shifters that require high power, and are range limited to frequencies, greater than a few megahertz, that satisfy the Bragg condition. Recent applications involving acousto-optic phenomenon either replace the Bragg acoustic wave with a travelling, electro-optically-induced index grating, to allow Bragg diffraction, at a fixed angle, down to arbitrarily low frequencies, or achieve frequency shifting by Bragg scattering from a collinear electro-optically induced traveling wave of off-diagonal polarizability, which couples TE and TM waveguide modes. An input optical wave couples to an orthogonally polarized wave at a different frequency. The wave velocity can be electrically adjusted; hence, the frequency shift is tunable. Optical bandwidth is related to the birefringence, N.sub.e -N.sub.o, of the waveguide material, and to overall device length. Optical bandwidth of ten (10) Angstroms, attainable in devices made of lithium niobate having a birefringence N.sub.e -N.sub.o =0.09, is too narrow for application in closed-loop phase nuling fiber optic gyroscopes, since superluminescent diode light sources of 100-150 Angstrom bandwidth are used. Obviously, broadband waveguide frequency shifters are required for such applications.